There are several known articles used to release an active substance in a controlled manner. Such articles are usually made up of a carrier layer and a control layer as disclosed in, for example, U.S. Pat. No. 4,160,335. The carrier layer contains an active substance dissolved therein. The control layer, which covers the carrier layer, controls the permeation or diffusion of the active substance so that the active substance is released into the surrounding atmosphere at a substantially constant rate. When the active substance is a liquid, the carrier layer is prepared by uniformly dissolving the active substance in a polymer which is highly miscible with the active substance. Thus, the prepared carrier layer is covered by the control layer comprising a polymer which is less miscible with the active substance.
However, the above articles are disadvantageous in that the solubility of the active substance in the polymer used as the carrier layer is limited. Thus, it is usually difficult to have the carrier layer contain more than 30 wt % of the active substance. If a liquid active substance is dissolved in the polymer in an amount greater than 30 wt %, the polymer cannot form a solid matrix. Therefore, the content of the active substance in such articles is usually limited to about 10 wt %.
In order to overcome the above-disclosed disadvantage, a carrier layer which is composed of an open-cell porous polymer membrane and an active substance absorbed in the membrane has been used.
However, the above carrier layer is disadvantageous in that the surface of the carrier layer is made wet by the active substance. This makes it difficult to form a control layer thereon. Therefore, the carrier layer is usually enclosed in a membrane body which serves as the control layer. However, the use of membrane body is disadvantageous since it requires additional production steps which increases the cost of the articles.